In the fabrication process for integrated circuit devices, various chemicals including gases and liquids are utilized. One of the gases utilized as a reactant in a silicon oxide deposition process or as a cleaning agent for a wafer surface is ozone. For instance, silicon oxide films can be deposited onto semiconductor wafers at atmospheric pressure and at low deposition temperatures by reacting tetraethoxysilane (TEOS) with ozone. Ozone is used as an reactant to produce films which exhibit smooth profiles over steps and therefore is suitable for filling high aspect ratio gaps between metal lines. Silicon oxide films produced by ozone and TEOS is most suitable as inter-metal dielectrics.
Ozone has also been used as an effective cleaning agent for wafer surfaces. For instance, after a photolithographic process, a photoresist layer can be stripped by the combination of a dry ashing process with ozone and then a wet cleaning process with a mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2. In the process, most of the organic residue from the photoresist can be removed in the ozone ashing process while wet cleaning is used to render the wafer surface completely clean. In another process that utilizes ozone for cleaning, ultrapure water injected with ozone can be used to clean wafer surfaces. When ozone is first dissolved in ultrapure water, ozone decomposes and becomes a strong oxidizing agent capable of decomposing organic impurities. The ozone-injected ultrapure water cleaning process therefore provides the advantages of a lower cleaning temperature, a simplified process, and a reduced chemical consumption. There is, however, a side effect with the ozone cleaning process in that native oxide may grow on the wafer surface at high ozone concentrations. After an ozone cleaning process is completed on a wafer surface, the organic contaminant-free surface resulting from the ozone treatment further helps subsequent cleaning steps to function properly. Overall, the ozone-injected ultrapure water cleaning process is an effective method to remove all organic impurities on a wafer surface. It can be carried out at room temperature and can be used to replace a conventional H.sub.2 SO.sub.4 /H.sub.2 O.sub.2 wet cleaning process.
Ozone is a triatomic allotrope of oxygen which has a characteristic pungent odor. Ozone is produced naturally in the earth's stratosphere by the absorption of solar radiation into oxygen. Ozone is also present in the earth atmosphere in low concentration as a consequent of intrusions of stratospheric air. Since ozone exists in an unstable state, it decomposes into oxygen at normal temperature and pressure. Such characteristic enables ozone to be a powerful oxidizing agent. Its strong ability to oxidize has been utilized in the fabrication processes for integrated circuit devices whenever an oxidation process is desired.
In a semiconductor fabrication facility, ozone is normally generated by a static discharge method such that a large quantity of ozone in high concentration can be produced for production use. In the static discharge technique, an oxygen gas is passed through inbetween two electrodes which are coated by a ceramic dielectric material and are separated by a narrow gap formed inbetween. The electrode arrangement is known as a discharge cell. The reaction to form ozone can be initiated when a voltage is applied to the discharge cell. Oxygen molecules are decomposed into oxygen atoms from collisions between the electrons and the oxygen molecules. The active oxygen atoms then recombine with surrounding oxygen molecules to form ozone. The reaction can be expressed as 3 O.sub.2 .fwdarw.2 O.sub.3. The ozone synthesis process proceeds in an equilibrium chemical reaction. The reaction rate increases as the reaction temperature is increased. Since most of the energy applied to the discharge cell is converted to heat and that if heat is not removed, the ozone produced will be destructed at the high temperature. As a consequence, the discharge cell for ozone production must be efficiently cooled by a heat exchanger.
In order to supply a large enough volume of ozone for production use in a semiconductor fabrication plant, a series of ozone generating units (each unit in turn consists of a multiple number of generating cells) are connected together in parallel so that a high concentration and large volume of ozone can be produced for supplying to a deposition or cleaning process . With the increasing number of ozone generating units used, the chances of having ozone leaks from one or more of the units become significantly higher. When ozone leaks from a generating unit occur in a semiconductor fabrication plant, several problems can result due to the leakage. First, since ozone breaks down easily into oxygen at normal temperatures and pressures, and oxygen helps combustion of many flammable materials which are used in a semiconductor fabrication plant, ozone leakage presents a serious fire and explosion hazard. Secondly, the inhalation of ozone into human body produces a various health hazard that may be detrimental to the machine operators. Thirdly, Ozone has an unpleasant, pungent odor that is objectionable to most people. Unfortunately, commercially available ozone generators or generating units are not equipped with leakage detectors which can be used to effectively detect ozone leakage and thus enable an operator to correct the problem.
In addition to the need of detecting ozone leakage at a generating unit by installing flow sensors attached to the inlet and outlet of the unit, it is also desirable to design a system that takes remedial actions when leakage is detected in one or more generating units. The use of a main controller is therefore desirable which takes into account the loss of ozone production from the generating units that are shut down by increasing the output of the other ozone generator units so that the total output is maintained and the fabrication process is not affected.
In addition to the capabilities of detecting a leakage in an ozone generating unit, and of taking remedial actions for maintaining a constant output of zone, it is further desirable that other potential leakages that does not occur in a generating unit can also be detected and that remedial actions can be taken to stop the leakage. For instance, potential leakages may occur in the piping system that connects the generating units together to a main reactant gas input line and a main product gas output line. Such leakage if not detected can also cause severe problems to the fabrication process.
It is therefore an object of the present invention to provide a gas generating device equipped with a gas leakage detection and control system that does not have the drawbacks or shortcomings of the conventional gas leakage detection system.
It is another object of the present invention to provide a gas generating device equipped with a gas leakage detection and control system that is not only capable of detecting leakages from individual generating units but also capable of detecting leakages in the piping system that connects the individual generating units.
It is a further object of the present invention to provide a gas generating device that is equipped with a gas leakage detection and control system having a series of sub-controllers for controlling each individual generating unit and also a main controller for controlling the complete system.
It is another further object of the present invention to provide a gas generating device that is equipped with a gas leakage detection and control system that is capable of shutting off individual generating unit when a leakage is detected in such unit and then through the operation of a main controller, to increase the output of the other generating units such that the total gas output from the generating system is not affected.
It is still another object of the present invention to provide a gas generating device that is equipped with a gas leakage detection and control system that is capable of shutting off the whole system when a leakage is detected in the piping system that connects the individual generating units by detecting and comparing a flow rate in a main gas inlet and a flow rate in a main gas outlet.
It is yet another object of the present invention to provide a gas generating device that is equipped with a gas leakage detection and control system that utilizes a multiple number of subcontrollers and a main controller to operate air actuated valves for opening or closing of gas inlets and gas outlets to each generating unit and a main gas inlet and a main gas outlet for the gas generating device.
It is still another further object of the present invention to provide a method for detecting an ozone leak from an ozone generator that is capable of detecting not only leaks in each ozone generating unit, but also leaks in the piping system that connects the generating unit.
It is yet another further object of the present invention to provide a method for detecting an ozone leak from an ozone generator that is not only capable of detecting leaks from individual ozone generating units but also capable of taking remedial actions by shutting down the leaking unit while increasing the outputs from the other generating units such that the total output from the ozone generator is maintained.